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Caption

Radio telescope. Parabolic open-mesh dish of a radio telescope, used to detect radio waves from space. Radio waves are emitted by objects such as nebulae (clouds of gas and dust), and non-luminous objects in distant galaxies. Radio waves can be detected by ground-based astronomy because Earth's atmosphere is transparent to radio waves. Optical mirrors cannot be used to collect radio waves. Instead, a mesh of metal wires is used to reflect the radio waves towards the detector at the focus of the dish. The spacing between the wires has to be small enough for the wavelength being observed. A picture is not obtained instantly, but built up by tilting the dish to scan the observed object.